1. Field of the Invention
The present invention relates to a photoelectric conversion device wherein a photoelectric conversion layer composed of a crystalline silicon film is formed on a substrate, and a process for producing the same.
2. Related Art
A photoelectric conversion device can be produced using any one of various semiconductor materials and organic compound materials. However, the photoelectric conversion device is industrially produced using silicon mainly. The photoelectric conversion device using silicon can be classified into a bulk type photoelectric conversion device using a wafer made of monocrystal silicon or polycrystal silicon and a thin film type photoelectric conversion device wherein a silicon film is formed on a substrate. For the bulk type photoelectric conversion device, a semiconductor substrate (such as a silicon wafer) is necessary in the same way as for a LSI (large-scale integrated circuit). The production amount thereof is limited by the supply capacity of the semiconductor substrate. On the other hand, it is considered that potential production capacity of the thin film type photoelectric conversion device is higher because of the use of a semiconductor film on a given substrate.
At present, a photoelectric conversion device using amorphous silicon is made practicable. However, this photoelectric conversion device has lower conversion efficiency than the photoelectric conversion device using monocrystal silicon or polycrystal silicon. Furthermore, this photoelectric conversion device has problems such as deterioration by light. Thus, the use of this photoelectric conversion device is limited to products having a small power consumption, such as a pocket calculator. For sunshine power generation, photoelectric conversion devices using a silicon film obtained by crystallizing an amorphous silicon film (the obtained silicon film being referred to as a crystalline silicon film hereinafter) have been actively developed.
The method of forming the crystalline silicon film is classified into melting recrystallization and solid phase growth methods. In both the methods, amorphous silicon is formed on a substrate, and this silicon is recrystallized to form a crystalline silicon film. In either case, the substrate is required to endure the crystallization temperature of silicon. Thus, the material which can be used for the substrate is limited. Particularly in the melting recrystallization method, the material for the substrate is limited to a material enduring the melting point of silicon, that is, 1412xc2x0 C.
The solid phase method is a method of forming an amorphous silicon film on a substrate and then subjecting the film to heat treatment to crystalline the film. Usually, the amorphous silicon film is hardly crystallized at a temperature of 500xc2x0 C. or lower. Practically, it is necessary to heat the amorphous silicon film at 600xc2x0 C. or higher. For example, in the case that an amorphous silicon film formed by a vapor growth method is heated to be crystallized, a heating time of 10 hours is necessary when heating temperature is 600xc2x0 C. When the heating temperature is 550xc2x0 C., a heating time of 100 hours or more is necessary.
For the reasons as described above, the substrate for forming a crystalline silicon film is required to have high heat-resistance. It is therefore preferred to use quartz, carbon, a ceramic material or the like as the material for the substrate. However, such a substrate is not necessarily suitable for a reduction in production costs. It would be primarily desired that an inexpensive material circulated in a great amount in the market is used as the material for the substrate. However, for example, a #7059 glass substrate made by Corning Incorporated, which is in general frequently used, has a strain point of 593xc2x0 C. Thus, if a conventional crystallizing technique is used, this substrate is distorted to generate large deformation. Therefore, the substrate is not used. Since the substrate is made of a material essentially different from silicon, a monocrystal film cannot be obtained even if heat treatment for crystallization is merely performed. As a result, only a polycrystal film can be obtained. The grain size of the polycrystal film is not easily made large. This fact results in the suppression of an improvement in the efficiency of photoelectric conversion device.
As a method for solving the above-mentioned problems, JP-A-7-58338 discloses a technique wherein a very small amount of a catalyst element is added as a catalyst material for the promotion of crystallization at low temperature, thereby attaining the crystallization. According to this official gazette open to the public, it becomes possible to make heat treatment temperature low and make treatment time short. For example, in the case that the heating temperature is set to 550xc2x0 C., it is verified that silicon is crystallized by heat treatment for 4 hours. The official gazette states that a single element of nickel (Ni), iron (Fe), cobalt (Co) or platinum (Pt), a compound of any one of them and silicon, or the like is suitable for the catalyst element.
Originally, however, all of the catalyst materials used to promote the crystallization are materials unpreferable for crystalline silicon. It is therefore desired that the concentration of the catalyst material is as low as possible after the crystallization. The concentration of the catalyst material necessary for promoting the crystallization is a range from 1xc3x971017 to 1xc3x971020/cm3. However, even if the concentration is relatively low, the element suitable for the catalyst material, when taken in silicon, generates a defect level because the element is a metal. Thus, it is evident that this defect level causes the deterioration of important characteristics for a photoelectric conversion device, such as the lifetime of carriers.
Incidentally, it can be considered that the outline of the action principle of a photoelectric conversion device produced by forming a PN junction is as follows. The photoelectric conversion device absorbs light, and generates carriers (i.e., electrons and holes) by the energy of the absorbed light. About the generated carriers, the electrons move toward its n layer and the holes move toward its p layer by drift and diffusion based on an electric field. In the case that silicon has many defect levels, the carriers are trapped into the defect levels on their way to become extinct. That is, the photoelectric conversion characteristic of the photoelectric conversion device deteriorates. The time from the generation of the electrons and holes to the extinction thereof is called a lifetime. It is desired that this value is larger for the photoelectric conversion device. Therefore, it is necessary that the amount of impurity elements, which generate the defect level in silicon, are originally as small as possible.
An object of the present invention is to provide a photoelectric conversion device wherein good use is made of the advantage of the crystallization of silicon resulting from the above-mentioned catalyst material and further the catalyst material which is unnecessary after the crystallization is removed to exhibit a superior photoelectric conversion characteristic.
In order to solve the above-mentioned problems, the process for producing a photoelectric conversion device of the present invention comprises the steps of generating, for a semiconductor film having a crystal structure formed by adding a catalyst element for promoting crystallization to a semiconductor film having an amorphous structure, a strain field by means of a semiconductor film to which a rare gas is added or a semiconductor region to which a rare gas is added, as a means for removing the catalyst element remaining in the semiconductor film having the crystal structure; and using the strain field as a gettering site to segregate the catalyst element into this region.
That is, the process for producing a photoelectric conversion device of the present invention comprises the steps of forming a first semiconductor film having an amorphous structure; adding a catalyst element for promoting crystallization to the first semiconductor film having the amorphous structure; conducting a first heat treatment to form a first semiconductor film having a crystal structure; forming a second semiconductor film containing a rare gas element on the first semiconductor film having the crystal structure; conducting a second heat treatment to segregate the catalyst element into the second semiconductor film; and removing the second semiconductor film.
By adding, to the first semiconductor film having the amorphous structure, the catalyst element for promoting the crystallization thereof and then subjecting the resultant semiconductor film to the first heat treatment, heating temperature necessary for the crystallization can be made lower than in the prior art. The catalyst element(s) that can be used is/are one or more selected from Fe, Ni, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.
The catalyst element remaining in the first semiconductor film after the crystallization can be moved into the second semiconductor film and concentrated/collected by forming the second semiconductor film containing the rare gas element on the first semiconductor film and then conducting the second heat treatment. That is, by incorporating the rare gas element into the second semiconductor film, a strain field can be generated so as to be a gettering site. Since the rare gas element is not basically bonded to another atom, the rare gas element is inserted between lattices in the semiconductor film, thereby generating the strain field.
The gettering technique is well known as a technique for producing an integrated circuit using a silicon monocrystal substrate. As the gettering technique, the following are known: extrinsic gettering, wherein a strain field or a chemical effect is supplied to a silicon substrate from the outside so as to generate gettering effect; and intrinsic gettering, wherein a strain field based on lattice defects with which oxygen generated inside a wafer is concerned is used. Examples of the extrinsic gettering include a method of giving mechanical damage to the back face (that is, the face opposite to the face on which elements are to be formed) of a silicon substrate, and a method of forming a polycrystal silicon film, and a method of diffusing phosphorus. There is also known a gettering technique performed in the state that a strain field is generated by secondary lattice defects formed by ion implantation. The detailed mechanism of the gettering has not been necessarily made clear. However, the following phenomenon is positively used in the mechanism: when heat treatment is conducted as described above, metal elements are precipitated in the region where a strain field is generated.
In order to remove the second semiconductor film formed on the first semiconductor film selectively after the gettering is performed, it is advisable to form a barrier layer on the first semiconductor film. The barrier layer may be formed by treating the first semiconductor film with ozone water to form a chemical oxide, by treating the first semiconductor film with plasma to oxidize the surface thereof, or by radiating ultraviolet rays in an atmosphere containing oxygen to generate ozone and oxidizing the surface with ozone.
The second semiconductor film is formed by sputtering or plasma CVD. A rare gas element can be taken in the second semiconductor film by incorporating the rare gas into the sputtering gas or adding the rare gas to the reaction gas. After the formation of the film, the rare gas may be added by ion implantation or ion doping. As the rare gas, a gas selected from He, Ne, Ar, Kr and Xe is used.
The first heat treatment and the second heat treatment are conducted by rapid thermal annealing (RTA) using a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressure sodium lamp, or a high-pressure mercury lamp as a heating means or by furnace annealing. In the present invention, it is sufficient that the gettering causes the catalyst element to move at a distance corresponding substantially to the thickness of the semiconductor film. Thus, the gettering can be completely accomplished even by short-time heat treatment such as RTA.
According to the present invention, the second semiconductor film wherein a strain field is generated by the addition of a rare gas is used as a gettering site; therefore, the layer at the incident side of light in the photoelectric conversion device can be formed as an n-type semiconductor layer or a p-type semiconductor layer. This makes it possible to select the substrate on which the semiconductor film having a crystal structure is formed from various substrates, and select the layer at the incident side of light freely from both of n-type and p-type semiconductor layers.